Esther Inglis-Arkell

One of the main reasons humans are the animal equivalent of gas-guzzling SUVs is our giant freak brains. Our grey matter burns a lot of energy, and that means we need a lot of food to fuel them. But if that's the case, why aren't you losing weight just by sitting at your desk? Last time we checked, most work-out programs don't involve complex mathematics or transcendental meditation.

So how many calories does using your brain actually burn?

Different types of documentaries have different conventions. Wildlife documentaries feel compelled to show two of nature's least attractive creatures mating. Historical documentaries are going to find a way to work World War II in, no matter what. And documentaries about human evolution are going to have at least one section in which they talk about the expanding human brain.

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These documentaries always show the line up of skulls as brains got bigger, and they always — always — talk about how our large brains must have had an evolutionary purpose, because they sucked down energy like crazy. But the explanations are always somewhat different. When we were capable of eating hunting meat with tools, we could feed our expanding brains. When we started cooking food, the expanded amount of calories we got from it allowed us to feed our energy-hungry brains. And the virtuous cycle of bigger brains and better food continued.

So why don't we have any mental aerobics videos? Why don't computer programmers (or, say, bloggers) have the bodies of Olympians? Did all those documentaries lie to us? How many calories do people burn by using their brains, and can that amount change?

To get an idea of the calories that anyone burns at any time, the entire walking process burns about four calories per minute, above and beyond the 1300 calories a day that most people need to stay alive. Kickboxing, that favorite activity of cage-fighters, burns about 10 calories a minute. Now the brain, all on its own, burns only about a tenth of a calorie per minute. Percentage-wise, that's very high for a resting body tissue. When the brain kicks into gear, though, it really gets impressive. When actively thinking about things, the brain can kick it up to burning a calorie and a half per minute. Considering it's an inert mass of goo that makes up only two percent of a person's body weight, that's impressive.

What's more impressive, though, is a team of scientists found a way to measure when, and how much, a person's brain is thinking. Neurons work by producing and giving off neurotransmitters. These are chemicals that are produced and given off by one neuron. The next neuron takes up the transmitter, and through these signals get past through the nervous system. In order to produce and transmit these transmitters, neurons suck up twenty-five percent, sometimes more, of the total body glucose, and then twenty percent of oxygen from the bloodstream. It gets the sugar in the form of glucose. During a PET scan, people can see the rate of glucose uptake of different parts of the brain. The frontal lobe is what really needs to be engaged while someone is thinking.

Unfortunately, it can't be engaged all the time. While a brain takes up about twenty percent, or 300, of a resting body's 1300 calories a day, and while it has the potential to burn more, it's estimated that most actual thinking only changes the amount of calories that the brain burns by around twenty to fifty calories per day. That tells us a few things about how much time we spend thinking. Given the shape we stay in today, though, we probably won't think too hard about that.